Air strain mechanism for bandmill

ABSTRACT

Bandmill strain mechanism having low friction and inertia losses. Bandmill has two spaced coplanar wheels carrying endless saw, first wheel moveable relative to second wheel to strain saw. First wheel arbor carried in carrier arm hinged at one end to yoke assembly. Air spring extends between opposite end of arm and yoke assembly and resiliently mounts arbor to absorb fluctuations in load. Extensible jack moves yoke assembly to strain saw, guide preventing rotation of yoke assembly. Yoke assembly has provision for alignment of wheels.

United States Patent Baldrey et al.

AIR STRAIN MECHANISM FOR BANDMILL Inventors: Robert Alexander Baldrey, West Vancouver, British Columbia; Francis Edwin Allen, North Vancouver, British Columbia, both of Canada Letson and Burpee Ltd., Vancouver, British Columbia, Canada Filed: May 29, 1973 Appl. No.: 365,021

Related US. Application Data Continuation-in-part of Ser. No. 320,535, Jan. 2, 1973, abandoned.

Assignee:

US. Cl. 83/819 Int. Cl B27b 13/08, B23d 55/10 Field of Search 83/817, 818, 819

References Cited UNITED STATES PATENTS 8/1905 Cleveland 83/819 1 Oct. 1, 1974 Felton 83/818 Crane 83/819 Primary Examiner-Donald R. Schran Attorney, Agent, or Firm-Carver and Company [57] ABSTRACT Bandmill strain mechanism having low friction and inertia losses. Bandmill has two spaced coplanar wheels carrying endless saw, first wheel moveable relative to second wheel to strain saw. First wheel arbor carried in carrier arm hinged at one end to yoke assembly. Air spring extends between opposite end of arm and yoke assembly and resiliently mounts arbor to absorb fluctuations in load. Extensible jack moves yoke assembly to strain saw, guide preventing rotation of yoke assembly. Yoke assembly has provision for alignment of wheels.

19 Claims, 10 Drawing Figures PATENIEUnm um 18385620 smears PATENIEB SHEET 30$ 5 PAIENIEBnm 11914 v SHEET sor s' law AIR STRAIN MECHANISM FOR BANDMILL CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation-in-part application of our copending application Ser. No. 320,535 filed .Ian. 2, 1973 in BANDMILL, now abandoned.

BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to a bandmill, in particular to a strain mechanism for loading a saw of the bandmill.

2. Prior Art A bandmill has a pair of spaced co-planar wheels, the wheels carrying a bandsaw. A force from a strain mechanism forces the wheels apart and applies a particular stress to the bandsaw, hereinafter bandsaw strain, so as to apply tension to the saw, commonly known as straining the saw.

In many bandmills, the strain mechanism uses a dead weight lever mechanism operating through a knife edge arrangement. This type of mechanism has been used successfully for many years, but it has inherent limita 'tions as below. Fluctuations in the cutting load produce excessive tension in the saw such that life of the saw is reduced, and quality of cutting is impaired. Excessive tension appears to result from slow response of the strain system to accommodate rapidly applied additional forces on the saw resulting from, for instance, lateral deflection of the saw. Slow response of the system is attributed to friction occuring at the knife edges and other bearing points and inertia of the weighted lever arm. High strain bandmills produce smaller kerfs and more accurate cuts, but undesirable effects of friction and inertia are accentuated in such high strain bandmills.

Four major parameters of the performance of a bandmill are outlined below.

i. When a saw is cutting through wood irregularities, such as knots, deflect the saw out of a cutting plane, such deflection subjecting the saw to an increased local strain. Such deflection is impulsive, i.e., applied for a short period of time, and reduces accuracy of cut and life of the saw. Lateral displacement of the saw is fed back through the strain system and results in movement of the lever arm at an interval of time after applying the lateral force to the saw. Friction and inertia inherent in the strain system tend to increase the time interval between application of the lateral force, hereinafter applied force, and initial response of the strain arm. This time interval determines speed of response of the strain mechanism. The faster the speed of response of a strain system, the more capable the system is of maintaining blade stability.

ii. Time required for the blade to return to its equilibrium level of stress after an impulsive load has been applied is defined as damping time, and this reflects natural response of the blade-bandmill system as a function of time.

iii. If additional strain energy is given to a stressed bandsaw and then removed, it is found that only a fraction of the additional energy is recovered. The remainder of the energy is lost in friction, such loss herein defined as hysteresis energy loss. Generally speaking a strain system in which metallic springs are deflected has a high hysteresis energy loss. whereas a system in which a fluid is compressed has a low hysteresis energy loss. These losses are inherent in the work done in applying a force in such systems.

iv. A further measure of a strain system is measurement of the force required to overcome static friction. This refers to a minimum additional force required to give a detectable increase of unit strain on one side of .the saw, in response to small increases in strain applied to the strain mechanism. Such force reflects a measure of sensitivity of the strain system to variation in indicated and applied strain.

The four paramaters above are dependent, to some extent, on friction and inertia of the strain mechanism. Thus if friction and inertia can be reduced an improvement in the strain mechanism would result.

SUMMARY OF THE INVENTION The invention reduces difficulties of the prior art strain mechanisms by providing a bandmill with a strain mechanism in which effects of inertia and friction are considerably less than in prior art strain mechanisms. The deadweight lever mechanism and knife edge arrangement are eliminated and force is applied through an air spring, later particularized.

One embodiment of the bandmill has spaced coplanar first and second wheels adapted to carry and drive an endless saw, the wheels mounted for rotation about axes co-planar with a central longitudinal plane normal to diametrical planes of the wheels. The bandmill has a column supporting a strain mechanism. the mechanism moving the first wheel within the longitudinal plane relative to the second wheel to strain the saw. The strain mechanism includes a yoke assembly, a jack, a carrier arm and an air spring. The yoke assembly has a hinge portion and a guide slideable within the column.

A restraining means cooperates with the yoke assembly to limit movement of the yoke assembly to motion parallel to the longitudinal plane with negligible rotation relative to the column. The jack extends between the column and the yoke assembly and moves the yoke assembly relative to the column, for instance when changing the saw. The jack has a central axis within the longitudinal plane of the bandmill to reduce bending forces on the carrier arm. The carrier arm carries the arbor of the first wheel and has a hinge portion complementary to the hinge portion of the yoke assembly. The hinge portions form a hinge to journal the carrier arm to the yoke assembly to permit rotation of the arm relative to the yoke assembly. The air spring extends between the carrier arm and the yoke assembly and communicates with a pressurized gas supply, the air spring serving as an extensible and retractable means to permit rotation of the carrier arm to strain the saw and to provide an essentially frictionless resilient mounting to accommodate fluctuations in saw loading.

In one embodiment, the air spring has a central datum plane when in a mid-stroke position. The hinge has a central axis about which the carrier arm rotates and the first wheel arbor has a central axis about which the wheel rotates. When the air spring is at a mid-stroke position, the central axes of the hinge and arbor are within the central datum plane of the air spring.

A detailed description following, related to drawings, described a preferred embodiment of the invention,

structure particularly described and illustrated.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmented side elevation of a bandmill according to the invention. some portions removed to show interior detail,

FIG. 2 is a fragmented perspective of an upper wheel of the bandmill of FIG. 1,

FIG. 3 is a fragmented perspective in section showing portions of one embodiment of an air spring associated with the bandmill,

FIG. 4 is a fragmented diagram showing upper wheel arbor raising and tilting means,

. FIG. 5 is an alternative horizontal bandmill according to the invention,

FIG. 6 is a fragmented simplified side elevation of a portion of an alternative strain mechanism according to the invention, showing an alternative embodiment of an air spring,

FIG. 7 is a simplified fragmented top plan view of the alternative mechanism of FIG. 6;

FIG. 8 is a simplified fragmented section on 8-8 of FIG. 7, showing a portion of an alternative hinge.

FIG. 9 is a fragmented perspective of an alternative arbor key means,

FIG. 10 is a fragmented diagram of an alternative restraining means of a plunger.

DETAILED DISCLOSURE FIG. 1

A bandmill 10 according to the invention has an upper wheel 11, a first wheel, and a lower wheel 12, a second wheel, the wheels being adapted to carry and drive and endless saw 14, shown as a broken line. The upper and lower wheels are mounted for rotation about arbors 16 and 17 respectively, the arbors having undesignated axes coplanar with a central longitudinal plane of the bandmill which is normal to diametrical planes of the wheels. The central longitudinal plane is shown as a broken line 19 which intersects centres of the arbors 16 and 17. The lower wheel 12 is driven by a motor 21 through common drive means 22shown in broken outline. Common saw guides 24 and 25 cooperate with the saw 14 to maintain the saw within a selected cutting plane, lumber to be cut (not shown) passing through the saw plane in a direction parallel to the plane 19. The bandmill has a column 27 extending from a base 29, the column having clearance for the upper wheel at an upper end 30. Thus portions of the upper end 30 are spaced sufficiently to straddle the wheel 11, as shown in FIG. 2, and the bandmill is essentially symmetrical about a central vertical plane normal to the plane 19 and co-planar with central diametrical planes of the wheels 11 and 12.

The bandmill 10 has a strain mechanism 32 cooperating with the wheel 11, the mechanism being adapted to move the wheel 11 within the longitudinal plane relative to the second wheel to strain the saw. The mechanism 32, to be described in more detail with reference to FIG. 2, has two similar portions, a first portion 33 only is to be described. The portion 33 has a carrier arm 34 carrying the arbor 16 of the wheel 11, the carrier arm having a hinge portion to hinge the arm at one end to a yoke assembly 36, the arm having at an opposite end thereof an air spring 38. The air spring 38, later particularized, extends between the carrier arm 34 and the yoke assembly, the air spring being an extensible and retractable means to rotate the carrier arm relative to the yoke assembly so as to strain the saw and to provide an essentially frictionless mounting to accommodate fluctuations in saw loading.

A jack 39, mounted on the column 27 extends between the column and the yoke assembly. the jack being adapted to move the yoke assembly relative to the column for lowering and raising the wheel when changing the saw. A plunger 41 extends downwards from the yoke assembly 36 into a bore of the column 30, and is slideable within the column to limit movement of the yoke assembly to motion parallel to the longitudinal plane. An undesignated key secured to the plunger and a keyway in the column (not shown) essentially prevents rotation of the plunger relative to the column. The plunger has an axis 42 parallel to the longitudinal axis 19, the plunger being journalled in bearings severally 43 which permit accurate axial sliding of the plunger. The key thus serves as restraining means cooperating with the yoke assembly to limit movement of the yoke assembly to motion parallel to the longitudinal plane 19, with negligible rotation relative to the column. Means (not shown) to limit rotation of the arm 34 relative to the yoke assembly 36 are described with reference to FIG. 2.

The jack 39 has an undesignated central longitudinal axis which is coplanar with the longitudinal plane of the bandmill shown as the broken line 19. Thus force from the jack is applied in a plane of the arbor producing low bending forces on the yoke assembly with little tendency of the plunger 41 to bind in the bearings due to off-axis forces. If the jack axis were moved so that it was not directly beneath the arbor, there would be a tendency of the yoke assembly to rock about the jack, producing off-axis forces on the plunger 41.

The air spring communicates with a gas supply, a pressurized tank 45, through a pressure line 46, the line 46 passing through a central bore (not shown) of the plunger 41 and communicating with a port in the air spring, as shown in FIG. 3. A valve 47 is provided in the line 46 to permit closing of the line, and a gauge 48 indicates pressure within the tank 45, the gauge having an accuracy to within 0.2 psi. The tank is pressurized preferably with nitrogen gas to a maximum pressure of about 200 psi. and the opening of the valve 47 admits gas to the air spring 38. The tank 45 is coupled via a pressure regulator (not shown) to a pressurized gas supply such as a gas bottle. A thermostatically controlled heating device (not shown) maintains the tank 45 above ambient temperature so that an ambient temperature change does not require adjustment of air quantity to maintain desired pressure. FIG. 2

The arbor 16 is a dead arbor and has one end 51 carried in a bore of the carrier arm 34 as shown, and an opposite end 52 carrier in a bore of a similar carrier arm 54, the wheel 11 being disposed between the carrier arms. The carrier arm 54 is journalled in a yoke as' sembly 56 carried on a jack 57 and restrained to axial motion by a plunger 59. An air spring (not shown) similar to the air spring 38 extends between the yoke as sembly 56 and the carrier arm 54, for concurrently extending or retracting with the air spring 38 to prevent rotation of the carrier arms concurrently about the yoke assemblies. The carrier arm 54 and yoke assembly 56 are part of the second portion of the strain mechanism, which portion is essentially identical to, and is coupled so as to cooperate with, the first portion 33 and is not further described in detail.

The carrier arm 34 has inner and outer ends 60 and 61 respectively, the outer end cooperating with the air spring 38, the inner end being journalled on the yoke assembly. The yoke assembly 36 has a yoke carrier 62, the carrier 62 having the plunger 41 extending downwards therefrom into the bore of the column. The yoke carrier 62 cooperates with the jack 39 which extends between the column and the yoke assembly. The yoke assembly also has a yoke member 63 which has a pair of vertically extending hinge members 65 and 66, spaced so as to accept the inner end 60 of the carrier arm 34. The hinge members 65 and 66 support spherical bearings (not shown), the bearings journalling a hinge pin 67 extending through the inner end 60 and the bearings. The spherical bearings permit limited rolling of the carrier 34 about a longitudinal axis (not shown) simultaneously providing hinging about a horizontal axis (not shown) of the hinge pin which is essentially normal to the central longitudinal plane 19.

The yoke member 63 is releasably secured to the yoke carrier 62 by bolts 68 in threaded holes in the yoke member. Oval slots (not shown) in the yoke carrier accept the bolts and permit slight movement between the member 63 and the carrier 62 to provide an essentially continuous range of adjustment. The bolts and slots serve as adjustment means to permit movement of the yoke member relative to the yoke carrier for alignment adjustment of the arbor of the first wheel with the arbor of the second wheel.

As previously stated the carrier arm 34 has a bore which accepts the end 51 of the arbor. Pinch bolt means 70 cooperate with the bore to clamp the arbor rigidly in the bore of the carrier arm. A key 76 tits in complementary keyways in the bore of the carrier arm 34 and in the end 51 on the arbor, the key and keyways serving as key means to prevent rotation of the arbor relative to the carrier as in a conventional dead arbor. The key 76 is a relatively broad key, extending over approximately 60 of arc of the arbor so as to increase bearing area to reduce changes of slight relative movement between the key, the arbor, or the carrier arm. Thus the carrier arms and arbor form an essentially rigid Pl-shaped assembly capable of withstanding forces from the strain mechanism with negligible flexing or relative movement between the'arbor and carrier arms. A bridge 71 cooperates with the end 51 of the arbor 16 and has a pair of legs 73 and 74 urged into contact with a side wall of the carrier arm 34. A common screw means 75 extending through the bridge engages an end face of the end 51 and, in conjunction with a similar bridge and screw means (not shown) cooperating with the end 52 ot the arbor, is used for axial translation of the arbor for alignment of the wheels in the central vertical plane. Prior to such adjustment the pinch bolt means are loosened, and then retightened after adjustment. Other means to provide axial adjustment of the arbor 16 can be substituted. Thus the upper wheel can be adjusted for alignment and cross alignment as in a conventional bandmill.

The air spring 38 communicates with an upper end 78 of the pressure line 46 (not shown), which end passes through a bore (not shown) in an upper wall of the yoke member 63. As previously stated the line 46 passes through the central bore of the plunger 41 to communicate with the pressurized tank 45 (FIG. 1).

The means to limit rotation of the arm 34 relative to the yoke assembly includes a threaded shaft 80 passing through a bore in the arm 34. The shaft 80 has an inner I end 81 secured to the yoke member 63 and an outer end 82 carrying an adjustable stop, for example a pair of nuts screwed onto the end 82. The stop interferes with the arm 34 and limits upward rotation of the arm. Means to limit downward rotation can be a similar pair of nuts (not shown) or stop means secured to the yoke member. FIG. 3

The air spring 38 has a first portion 85 which is movable relative to a second portion 86, the second portion having a lower face 88 which is secured to an upper face of the yoke portion 63 (FIG. 2 only). The second portion 86 has an inner cylindrical side wall 90 forming a cylinder, and a threaded bore 89 forming an input port to accept the upper end 78 of the line 46. A lower end wall of the cylinder has a raised boss 91. The first portion has a piston 93 having coaxial inner and outer discs 95 and 96, the discs sandwiching therebetween an inner portion of a flexible diaphragm 98. The diaphragm 98 has an outer periphery retained between a Z-sectioned annular flange 100 secured to the portion 86, the diaphragm and cylinder defining a chamber within the air spring. A fold 102 of the diaphragm extends between the cylindrical wall 90 and the piston 93, the fold permitting axial movement of the piston relative to the cylinder along a central axis 103, which axis is a portion of an are as will be described. The piston 93 has a rod 105 having an inner end, and a bolt 106 securing to the inner end the discs, the diaphragm, and the boss 105 to form the piston. The rod 105 has an outer end 107 secured to the outer end 61 of the carrier arm so that rotation of the arm 34 moves the piston 93 within the cylinder. Clearances between the piston and cylinder are sufficient to accommodate arcuate movement of the piston resulting from rotation of the arm 34. The axis 103 is a portion of an are having a radius equivalent to the carrier arm 34 and is centred on the hinge pin 67 (FIG. 2). Movement of the piston is small compared to radius and thus the axis 103 is approximated to a straight line essentially parallel to the longitudinal plane of the bandmill.

The Z-sectioned flange 100 has an inner lip 112 and the outer disc 96 has an upward extending flange 113, the flange contacting the lip at an upper limit of travel of the piston 93. The lower limit of travel of the piston is reached when a head of the bolt 106 contacts the boss 91. Stroke of the piston 93 within the cylinder is about one inch which is adequate to accommodate fluctuations of the arm arising from changes in sawing forces.

To informthe operator of the position of the piston within the air spring, a remote piston position indicator (not shown) is provided for the air spring, for instance a potentiometer type of position indicator. The indicator can be divided into three-main zones defined by four points, a fifth point indicating a mid-position of the piston stroke. Outer upper and lower points define extreme upper and lower limits of travel of the piston i.e. when it contacts the stops. Inner upper and lower points define upper and lower limits of normal operating range of the piston, the inner points being selected to be on a linear portion of the spring characteristic,

and spaced equally about one-quarter of an inch from the mid point of piston travel. The outer points are preferably outside a range of movement of the arm 34, which movement is restricted by the upper and lower stops on the shaft 80. Thus the pistondoes not reach its upper and lower limits of travel as it is protected by the stops of the shaft 80.

Position of the arbor on the carrier arm, and the distance of the arbor and the air spring from the hinge is selected so that, for a particular size of bandmill, movement of one-half of an inch at the air spring causes the centre line of the arbor to move about one-eighth of an inch, which is within normal movement of the arbor whilst the saw is cutting. When air pressure is properly adjusted, there is little risk of the arm 34 contacting either top or bottom stops of the shaft 80 during normal operation. A convenient strain ratio has been found to be about 1 psi. of air pressure in the air spring provides a strain of about 250 pounds on the saw.

The piston and diaphragm thus produce a rolling diaphragm air cylinder, similar to that as supplied by the Bellofram Corporation, a corporation of Massachusetts, U.S.A.. Rolling diaphragm air cylinders supplied by the above corporation can be substituted for the air spring. Alternatively similar rolling diaphragm air cylinders, or rolling sleeve or bellows type of air spring, supplied by Firestone Industrial Products Company of Noblesville, Indiana, U.S.A. under the trademark Airide" can be substituted. Other equivalents are known and suitable types of air springs are selected to have means to accommodate arcuate movement of the arm 34.

FIG. 4

The jacks 39 and 57 are coupled to a mechanism that can perform two discreet movements, which movements are common to most bandmills and include:

i. raising and lowering of the upper wheel whilst maintaining the arbor 16 parallel with the lower wheel arbor, as in changing a saw and setting saw strain,

ii. raising and lowering one end of the arbor only, the other end being unchanged so as to tilt the arbor to maintain central tracking of the saw on the wheel.

The jacks 39 and 57 have jack shafts 120 and 121 driven through chain and sprocket combinations shown as broken lines 123 and 124 respectively. The chain and sprocket combinations 123 and 124 are driven from sprockets 126 and 127 mounted on a shaft 129 and extending transversely through the bandmill, centre line of the shaft only being shown. A hand wheel 131 is for use by an operator for fine adjustment of wheel position to attain a desired saw strain, and also for tilting the arbor. The wheel 131 has a manual clutch having a pin 132, which pin can be withdrawn from a recess in a clutch element 134 to disengage one jack drive.

Fine adjustment of wheel separation for setting the strain is effected by rotation of the hand wheel 131 with the pin 132 engaged in the recess so as to drive both jacks simultaneously. The upper wheel 11 is tilted by extracting the pin 132 from engagement with the clutch member 134, and rotating the wheel 131, which actuates only the jack shaft 121. A motor 136 drives the shaft 129 through a sprocket and chain combination 137 and is usedfor rapid lowering and raising of the wheels, for changing the saw. A further sprocket and chain combination 139 drives a rotary limit switch 140 which is in series with the motor so that when maximum or minimum limits of travel of the jacks is reached the motor is automatically stopped. This is to protect the jacks, and limits of travel of the jacks are outside limits of travel of the wheel. Other means of raising and lowering the wheel are known.

OPERATION The air spring 38 can be maintained at normal operating pressure when no saw is fitted on the mill. In such a condition the arm 34 is forced against the upper stop. When a saw is to be fitted on the band mill, the upper wheel 11 is lowered through the sprocket and chain assemblies driven by the motor 136. After fitting the saw on the wheels, the upper wheel is raised by the motor 136 and sprocket and chain combinations until an upper limit of travel of the wheel is approached. As tension is initially applied to the saw, the remote piston position indicator indicates when the piston starts to descend from an upper position. The indicator is provided with detectors which are actuated when the piston approaches a mid-position of its stroke at which the motor 136 is stopped automatically. The hand wheel 131 is then rotated with the pin 132 engaged in the clutch element 134 operating both jacks concurrently. Saw strain is increased until the piston 93 reaches the approximate mid-position of its stroke, as indicated by the indicator. The bandmill motor 21 can now be started and sawing commence. If saw wander is detected, the pin 132 is withdrawn from the clutch element 134 and the hand wheel rotated in such direction as to rotate the jack shaft 121 so as to tilt the wheel in such a manner as to correct the saw wander.

As previously stated, during cutting, fluctuations in load on the saw momentarily increase saw tension, resulting in small, essentially vertical movements of the arbor 16 about an equilibrium position. Initially the arbor moves downwards in response to the increase in load, for instance in response to a transverse movement of the saw due to a knot in the wood. This movement rotates the arm 34 about the hinge pin 67 which forces the piston 38 downwards in the air spring. The movement or response of the arm occurs rapidly after the increase in load as there is negligible friction and relatively low inertia. The friction loss occurs at the pin 67, friction losses in the air spring being negligible. Inertia of the wheel 11 and the arm 34 are relatively low as the counterweights are eliminated. The arm quickly returns upwards and settles in the equilibrium position, as there is little tendency for the arm to oscillate about the equilibrium position. Thus it has a short damping time, which is also attributed to a low inertia of the system due to elimination of the counterweights. Because the strain system uses a'compressed gas as a spring, hysteresis energy losses are relatively low when compared to a mechanical spring. The strain system has a high sensitivity to a small increase in strain. For example, the bandmill can be designed so that an increase of l psi. in the air spring can result in a theoretical increase in strain of 250 pounds, which is dependent on the strain ratio.

ALTERNATIVES AND EQUIVALENTS The bandmill as shown in FIGS. 1 and 2 has an upper wheel arbor carrying the upper wheel at an approximate midposition of the arbor, opposite ends of the arbor being carried in the carrier arms, the wheel being disposed between the carrier arms. An alternative design of bandmill utilizes an overhung wheel in which the upper wheel arbor is cantilevered from a carrier arm, the upper wheel being mounted adjacent an end of the arbor. With suitable modification, the strain mechanism according to the invention can be applied to an overhung wheel type of bandmill and obtain benefits of low friction and low inertia. Such types of bandmill require one portion of the strain mechanism only, that is one air spring and associated components. Difficulties associated with such an arrangement when used with high strain bandmills are well known in the trade, and particular care would be taken to ensure that the arbor and carrier arm would be sufficiently rigid to withstand the loads arising from the saw strain.

FIG.

A further alternative bandmill is a horizontal bandmill 145 shown schematically. The horizontal bandmill 145 has spaced coplanar first and second wheels 146 and 147 driving a saw 148. Saw guides 149 and 150 are provided adjacent a cutting portion of the saw as in a conventional horizontal bandmill. The first wheel 146 rotates about an arbor 151, the arbor carried in a carrier arm 153. An upper end of the carrier arm 153 is journalled in a yoke assembly 155 and the lower end cooperates with a counter-balanced air spring 157 extending between the yoke assembly and the carrier arm, similarly to the air spring 38 but in a position at 90 to the position as shown in FIGS. 1 and 2. A jack 158 and plunger 159 operate similarly to the jack 39 and plunger 40 of FIG. 1. Modifications required in this arrangement relate to the air spring which is counterbalanced and can operate with a central axis essentially horizontal.

FIGS. 6 and 7 An alternative strain mechanism according to the invention has a carrier arm 160 having inner and outer ends 161 and 162, the inner end having a hinge portion 163 which hinges the arm to an alternative yoke assembly 164. The yoke assembly has a yoke base 166 having an alternative plunger or guide 167 extending therefrom and a hinge portion 168 complementary to the hinge portion of the carrier arm, to be described with reference to FIG. 8. The hinge portions form a portion of a hinge 169 to journal the carrier arm to the yoke assembly to permit rotation of the arm relative to the yoke assembly. A jack 170 (broken outline), similar to the jack 39 of FIG. 1, extends between the column 27 (not shown) and the yoke assembly 164, the jack being adapted to move the yoke assembly relative to the column. The plunger is slideable within the column and has alternative restraining means to limit movement of the yoke assembly to motion parallel to the longitudinal plane with negligible rotation relative to the column, the alternative means to be described with reference to FIG. 10.

The carrier arm carries an alternative arbor 172 of the first or upper wheel. The first wheel in combination with a second or lower wheel carries a saw, the wheels and saw being similar to the wheels and saw 11, 12 and 14 respectively of FIG. 1 and are not illustrated. Similarly to the jack 39 of FIG. 1, the jack 170 has a central longitudinal axis 171 coplanar with the longitudinal plane 19-of the bandmill which, as aforesaid contains the central axes of the wheel arbors. The carrier arm has a bore 174 to accept one end of the arbor 172, al-

ternative pinch bolt means 175 cooperating with the bore 174 to clamp the end of the arbor rigidly in the bore. Alternative key means 176 cooperate with the bore 174 and the arbor 172 to prevent rotation of the arbor relative to the carrier arm and are described with reference to FIG. 9. Similarly to the arbor 16 the arbor 172 is a dead arbor and has a central axis 183 about which the wheel 16 rotates. A bridge and screw means 173 similar to the bridge 71 and means 75 of FIG. 1 provide alignment adjustment of the arbor 172 relative to the arm. An alternative stop means 177 has a bracket 178 extending from the arm, the bracket having a bore 179 to accept a stud 180 extending from the base 166. The stud has lock nut pairs providing upper and lower stops 181 and 182 which straddle the bracket to limit rotation of the carrier arm relative to the yoke assembly.

An alternative air spring 185 extends between the outer end 162 of the carrier arm and the yoke assembly and communicates with a pressurized gas supply (not shown) through an input port connected to a pressure line 187. The spring 185 serves a purpose similar to the air spring 38 FIG. 1 but differs structurally. The spring 185, the carrier arm 160, the jack 170 and the yoke assembly 164 are included in a first portion 188 of the alternative strain mechanism adapted tomove the first wheel within the longitudinal plane relative to the second wheel similarly to the portion 33 of FIG. 1. An essentially identical second portion (not shown) of the strain mechanism is coupled so as to cooperate with the first portion, the arbor 172 having each end carried in a respective portion, the first wheel being disposed between the respective carrier arms. The portions are coupled and actuated through a chain and sprocket mechanism similar to that shown in FIG. 4 for raising and lowering the upper wheel and for tilting the arbor 172.

The air spring 185 has a central axis 189, a first portion having a piston 190 and a second portion having a cylinder 192, clearance being provided between the portions so as to be moveable relative to each other with negligible friction losses. A flexible diaphragm 193 has an inner portion 194 secured to the piston and an outer peripheral portion 195 surrounding the inner portion and secured to the cylinder. A diaphragm fold 197 extends between the piston and cylinder to permit limited essentially axial movement between the portions with negligible diaphragm strain. The air spring is thus structurally similar to, and functions as, a rolling diaphragm air cylinder. The air spring has a central datum plane 200 defined by a portion of the piston when in a mid-stroke position as shown. The axis 183 of the arbor 172 is within the central datum plane of the air spring when at mid-position, for reasons to be explained.

The piston has a datum face 201 within the datum plane 200, the portion 194 of the diaphragm being secured and sealed to the face 201 by a cylinder cap 202 and screws. The cylinder is formed from two concentrically aligned rings 203 and 204 of equal bore, the rings being secured together at adjacent faces coplanar with the face 201. The outer peripheral portion 195 of the diaphragm is secured to and sandwiched between the adjacent faces above and is sealed to prevent gas leakage between the diaphragm and tne cylinder. The adjacent face of the ring 204 serves as a cylinder datum face 206 and thus the cylinder datum face surrounds the piston datum face and is generally coplanar with the piston face when the air spring is at mid-stroke position. The ring 203 is secured to the outer end 162 of the carrier arm, being keyed by a spigot 208 fitted in a bore of the end 162. The piston has a piston rod 210 having a bore 211 having inner and outer ends 212 and 213,

the inner end extending inwards from the piston face serving as the input port, and the outer end communicating with the pressurized gas supply through the line 187. The diaphragm has a bore 215 adajcent the central portion communicating with the end 212 and an undesignated bore in the cap 202, the bores admitting gas into a chamber 217 defined by the diaphragm and the cylinder. The rod 210 has a threaded outer end 219, and a threaded centering ring 220 having a cylindrical alignment portion 221 is screwed thereon. The ring is shown in full outline in a low position and in broken outline in a high position 220.1 A piston alignment plate 222 has a threaded bore to accept the end 219 and has a pair of opposed elongated slotted holes 223 and 224 shown in broken outline in FIG. 8, elongated portions of the slots being parallel to the arbor axis 183. The yoke base has a piston support 227 having a clearance bore .228 to accept the outer end 213 of the piston rod 210. The support 227 has a pair of similar elongated slotted holes 225 and 226, elongated portions of the slots being normal to the arbor axis 183 and portions of which are in register with portions of the holes 223 and 224. The plate 222 is secured to the support 227 by nut and bolt means 230 and 231 passing through portions of pairs of the slotted holes in register. The pairs of slotted holes cooperating with the means 230 and 231 permit the piston to be moved transversely relative to the cylinder in any direction normal to the axis 189 to provide adjustment for concentricity between the piston and the cylinder, to be explained.

The cylinder 192 has a rectangular cylinder end plate 234 secured by bolts 236 and 237 to the cylinder 192. The plate 234 has a bore 239 concentric with the axis 189 and having a diameter sufficient to accept snugly the alignment portion 221 when the ring 220 is in the position 220.1. As the alignment portion 221 is concentric with the rod 210, and the bore 239 is concentric with the cylinder, the rod can be centralized with the cylinder when the ring 220 is in the high position 220.1. The nut and bolt means 230 and 231 are loosened to permit transverse movement of the piston due to the slotted holes, so that the piston can be adjusted to be concentric with the cylinder. After adjustment the means 230 and 231 are tightened, thus securing the plate 222 to the support 227. The ring 220 is then screwed down to the low position permitting movement of the piston through the bore 239. Such movement is approximately axial but has a slight curvature due to rotation about the hinge 169.

An arm 242 having a pointer 243 extends from the cylinder and is adapted to sweep a scale 244 secured to the piston to indicate position of the piston within the cylinder relative to the datum plane of the air spring. This is an alternative piston position indicator. Limit switches (not shown) can be fitted to be actuated when the piston reaches extreme limits of travel, the limit switches operating warning lights fitted in an operators console (not shown).

FIG. 8 with references to FIGS. 6 and 7.

The hinge 169 has a hinge pin 249 having a central axis 250 and an eccentric cam 251 spaced between coaxial aligned outer ends 253 and 254 of the pin. The end 254 has opposed flats 255 providing means to turn the hinge pin about axis 250. The hinge portion 168 of the yoke base has two spaced hinge members 256 and 257 having aligned bores to accept the outer ends 253 and 254 respectively and being spaced sufficiently to straddle the cam 251. The hinge portion 163 of the carrier arm has a hinge bore 260 complementary to the eccentric cam, the hinge bore and cam journalling the arm.

The hinge bore 260 has an internal partially spherical bearing surface cnetered on an axis 263 and having a low friction coating, for example a coating of Teflon (trade mark), a suitable bearing being made by SKF of Schweinfurt, West Germany. The cam 251 has a par tially spherical bearing bush 262 having a bearing sur' face complementary to the hinge bore, the two surfaces providing a limited arc spherical bearing to permit tilting of the arbor. Such tilting is accomplished by raising one end of the arbor 16 relative to the other end to maintain central tracking of the saw on the wheel, by the jack actuating mechanism as shown in FIG. 4.

Referring to FIGS. 6 and 8, a spacing 265 between the axis 250 of the pin and the axis 263 of the spherical bearing of the cam defines throw of the hinge. The eccentric cam is shown in a centred position, termed centred hinge position, in which the axis 263 is spaced vertically above the axis 250 so that limited rotation of the pin 169 about the axis 250 in either direction moves the axis 263 generally horizontally relative to the axis 250, resulting in limited movement of the carrier arm generally normal to the central longitudinal plane 19 of the bandmill. The hinge thus has eccentric adjustment means to provide limited movement of the carrier arm to permit alignment of the wheel, and is an alternative to the limited movement between the yoke carrier 62 and yoke member 63 of FIG. 2. After such adjustment the piston rod requires centralizing relative to the cylinder. The carrier arm thus rotates about the axis 263. Note that when the hinge is centred, the axis 263 is disposed within the central datum plane 200 of the air spring for reasons explained below.

Thus when the hinge is centred and the piston is at mid-stroke position, the axis263 of the hinge 169 and the axis 183 of the arbor 172 are coplanar and within the central datum plate 200 of the air spring. In normal operation of the bandmill, essentially vertical movement of the arbor on an arc centred on the axis 263 results in a magnified similar movement of the cylinder in the air spring, which movement is unlikely to exceed about one-half of an inch. As the movement on the arc is small compared with radius of the arm it can be approximated to straight line movement. Such movement, due in part to the coplanar axes above, results in essentially equal movement of the fold 197 at all positions on the periphery of the fold, between upper and lower limits of piston travel, which approaches optimum operating conditions for the diaphragm. Rotation of the hinge pin for wheel alignment purposes results in a slight dropping of the hinge axis relative to the central datum plane 200, which dropping changes negligibly the above near optimum conditions.

FIG. 9.

As previously stated the arbor 172 is a dead arbor and is prevented from rotation relative to the carrier arm by the alternative key means 176. The key means 176 has a key 271 having a plane chord face 273 and a convex face 275, the faces being defined in part by common spaced parallel side edges 277 and 278. The face 273 is further defined by spaced parallel inner and outer end edges 281 and 282 and the face 275 is further defined by spaced parallel inner and outer end edges 283 and 284. Spacing between the side edges 277 and 278 defines chord length 286 and spacing between the end edges 281 and 282 defines axial length 287. The key is a segment of a cylinder having a radius generally equal to the radius of an outer end 289 of the wheel arbor 172 shown in broken outline. The key has a bracket 290 having a pair of spaced holes 291 and 292 to accept bolts (shown in FIG. 6) to secure the bracket to the carrier arm. The carrier arm has a recess adjacent the bore 174, the recess being complementary to the bracket 290 to cooperate with the bracket to prevent rotation of the key relative to the carrier arm. The outer end 289 of the arbor has an undesignated flat face having a chord length and an axial length generally equal to the chord length and axial length respectively of the key, the key being complementary to the outer end of the arbor so as to be within a cylindrical envelope of the arbor when the flat face of the arbor contacts the chord face 273 of the key. The cylindrical envelope enclosing the key and arbor is received in the bore 174 of the carrier arm so that when the key is secured to the carrier arm the arbor is restricted against rotation relative to the carrier arm. With the opposite carrier arm similarly secured to the arbor, a rigid H- sectioned assembly is formed.

An arrangement as above facilitates manufacturing of the arbor by eliminating a key way within the arbor and within the bore of the column, thus permitting relatively wide tolerances in fitting and yet still attaining a rigid assembly.

FIG. 10

As previously stated the plungers extending downward from the yoke bases are restricted against rotation relative to the columns. The plunger 167 is shown spaced from a corresponding plunger 298 of the second portion of the strain mechanism (not shown), the wheel 16 (broken outline) being spaced between the plungers which are journalled for axial movement in bearings, shown fragmented. An alternative restraining means 300 includes a connecting member 301 extending between lower end of the plungers, the member having a cranked portion 302 at a mid-position as shown. The member 301 is a flat strip which has sufficient stiffness in a horizontal plane to resist without buckling couples from either plunger, but has flexibility in a vertical plane to permit limited axial movement of one plunger relative to the remaining plunger so as to permit tilting of the first wheel arbor. The cranked portion provide sufficient accommodation to normal independent movements of the plungers and the key means cooperating with the plunger of FlG. 1 is eliminated. This simplities manufacturing and maintenance.

Operation of the bandmill equipped with the alternative arm 160 follows closely the operation of the bandmill equipped with the arm 34 of FIG. 1.

We claim:

1. A bandmill having spaced coplanar first and second wheels adapted to carry and drive an endless saw, the wheels mounted for rotation about axes coplanar with a central longitudinal plane normal to diametrical planes of the wheels, the bandmill having a column supporting a strain mechanism adapted to move the first wheel within the longitudinal plane relative to the second wheel to strain the saw, the strain mechanism having a first portion including:

a. a yoke assembly having a guide slideable within the column. and restraining means cooperating with the yoke assembly to limit movement of the yoke assembly to motion parallel to the longitudinal plane with negligible rotation relative to the column, the yoke assembly having a hinge portion,

b. a jack extending between the column and the yoke assembly. the jack adapted to move the yoke assembly relative to the column,

c. a carrier arm carrying the arbor of the first wheel,

the carrier arm having a hinge portion complementary to the hinge portion of the yoke assembly so as to form a hinge to journal the carrier arm to the yoke assembly to permit rotation of the arm relative to the yoke assembly,

d. an air spring extending between the carrier arm and the yoke assembly, the air spring communicating with a pressurized gas supply and serving as an extensible and retractible means to rotate the carrier arm relative to the yoke assembly to strain the saw, and to provide an essentially frictionless mounting to accommodate fluctuations in saw loading.

2. A bandmill as claimed in claim 1 in which:

i. the jack has a central longitudinal axis within the longitudinal plane of the bandmill, so that force from the jack is applied to the first wheel arbor in a plane of the arbor to reduce bending forces applied to the yoke assembly.

3. A bandmill as claimed in claim 1 in which i. the arbor is carried on the carrier arm at a position between the hinge and the air spring.

4. A bandmill as claimed in claim 1 in which i. the strain mechanism includes an essentially identical second portion coupled so as to cooperate with the first portion,

ii. the arbor has each end carried in a respective carrier arm, the first wheel being disposed between the carrier arms.

5. A bandmill as claimed in claim 1 in which i. the column has a bore having an axis parallel to the longitudinal axis of the bandmill,

ii. the guide is a plunger having an axis parallel to the longitudinal axis of the bandmill, the plunger siding in the bore of the column,

iii. the restraining means is a key secured to the plunger, the bore of the column having a complementary keyway, the key engaging the keyway to prevent rotation relative to the column,

iv. the air spring has an axis of extension and retraction, which axis is essentially parallel to the longitudinal plane of the bandmill. the air spring axis being spaced on a side of the arbor remote from the hinge of the yoke carrier.

6. A bandmill as claimed in claim 1 in which the yoke assembly has:

i. a yoke carrier having the guide, the yoke carrier cooperating with the jack extending between the column and the yoke assembly,

ii. a yoke member having the hinge portion, the yoke member being releasably secured to the yoke carrier and having adjustment means to provide limited movement of the yoke member relative to the and each carrier arm has:

yoke carrier to permit alignment of the arbor of the first wheel with the arbor of the second wheel.

7. A bandmill as claimed in claim 1 in which:

i. the hinge journalling the yoke assembly to the carrier arm has limited-arc spherical bearing to permit 5 tilting of the, arbor.

8. A bandmill as claimed in claim 1 in which the air spring has:

i. a first portion having a piston.

ii. a second portion having a cylinder, the portions being moveable relative to each other with negligible frictional losses,

iii. a flexible diaphragm having an inner portion secured to the piston, and an outer peripheral portion surrounding the inner portion and secured to the cylinder, the diaphragm and cylinder defining a chamber within the air spring, a fold extending between the piston and the cylinder to permit essentially axial limited movement between the portions with negligible diaphragm strain,

iv. an input port to conduct gas to the chamber.

9. A bandmill as claimed in claim 1 in which i. the strain mechanism includes an essentially identical second portion, the jack of which is releasably coupled to the jack of the first portion,

ii. the first wheel arbor has each outer end carried in a respective carrier arm, the first wheel being disposed between the carrier arms,

iii. inner and outer ends, the inner end being journalled on a respective yoke assembly by spherical bearings, the outer end cooperating with a respec tive air spring, a portion of the carrier arm between the ends having a bore to accept an outer end of the arbor of the first wheel,

iv. pinch bolt means cooperating with the bore of the carrier arm to clamp the respective outer end of the arbor rigidly in the bore of the carrier arm,

v. key means cooperating with the bore of the carrier arm and the arbor to prevent rotation of the arbor relative to the carrier arm,

10. A bandmill as claimed in claim 1 in which the yoke assembly has:

i. a yoke base having the hinge portion and the guid extending therefrom, the yoke base cooperating with the jack extending between the column and the yoke assembly,

ii. the hinge has eccentric adjustment means to provide limited movement of the carrier arm generally normal to the central longitudinal plane of the bandmill to permit alignment of the wheels.

11. A bandmill as claimed in claim 10 in which i. the hinge has a hinge pin having an eccentric cam spaced between co-axial aligned outer ends,

ii. the hinge portion of the yoke base has two spaced hinge members having aligned bores to accept outer ends of the pin and being spacedsufficiently to straddle the eccentric cam,

iii. the hinge portion of the carrier arm has a hinge bore generally complementary to the eccentric cam, the hinge bore and cam journalling the arm,

so that rotation of the hinge pin rotates the cam to move the hinge axis of the carrier arm relative to the hinge portion of the yoke base.

12. A bandmill as claimed in claim 11 in which:

i. the eccentric cam has a partially spherical bearing surface,

ii. the hinge bore of the carrier arm has an internal partially spherical bearing surface complementary to the eccentric cam to provide a limited arc spherical bearing to permit tilting of the arbor.

13. A bandmill as claimed in claim 1 in which:

i. the air spring has a central datum plane when in a mid-stroke position,

ii. the hinge has a central axis about which the carrier arm rotates, the central axis of the hinge being generally within the central datum plane of the air spring when the hinge is disposed centrally relative to the yoke assembly,

iii. the first wheel arbor has a central axis about which the wheel rotates, the central axis of the wheel arbor being generally within the central datum plane of the air spring when the air spring is at the midstroke position.

14. A bandmill as claimed in claim 8 in which:

i. the piston of the air spring has a piston datum face within a central datum plane of the air spring when at a mid-stroke position, the inner portion of the diaphragm being secured to the piston datum face,

ii. the cylinder of the air spring has a cylinder datum face surrounding the piston face and being generally co'planar with the piston datum face when the air spring is at the mid-stroke position, the outer peripheral portion of the diaphragm being secured to the cylinder datum face.

iii. the hinge has a central axis about which the carrier arm rotates. and when the hinge is centered so that the hinge is disposed centrally relative to the yoke assembly, the central axis of the hinge is generally within the central datum plane of the air spring,

iv. the first wheel arbor has a central axis about which the wheel rotates, the central axis of the arbor being within the central plane of the air spring when the air spring is at the mid-stroke position.

15. A bandmill as claimed in claim 14 in'which:

i. the piston has a piston rod having a bore having inner and outer ends, the inner end of the bore serving as the input port and extending inwards from the piston face, and the outer end of the bore communicating with the pressurized gas supply,

ii. the diaphragm has a bore adjacent the central portion communicating with the inner end of the bore of the piston rod to admit gas into the chamber defined by the diaphragm and the cylinder.

16. A bandmill as claimed in claim 9 in which the key means includes:

i. a key being a segment of a cylinder having a plane chord face and a convex face, the faces being defined by common spaced parallel side edges and spaced parallel inner and outer end edges, spacing between the side edges and end edges defining chord length and axial length respectively of the faces, the convex face having a radius generally equal to radius of the outer end of the wheel arbor. the key having a bracket adjacent the outer end the cylindrical envelope enclosing the key and arbor being received in the bore of the carrier arm so that when the key is secured to the carrier arm and the pinch bolt means is secured, the arbor is restricted against rotation relative to the carrier arm to form a rigid assembly. 5

edge of the face, the bracket cooperating with the carrier arm to prevent rotation of the key,

ii. an outer end of the arbor has a flat face having a chord length and an axial length generally equal to the chord length and axial length respectively of 5 the key, the key being complementary to the outer end of the arbor so as to be within a cylindrical envelope of the arbor when the flat face of the arbor contacts the chord face of the cotter piece,

the longitudinal axis of the bandmill,

iv. each guide is a plunger having an axis parallel to the longitudinal axis of the bandmill, the plunger sliding in the bore of the respective column,

v. the restraining means is a connecting member extending transversely between the plungers, the member following concurrent equal movements of the plunger and preventing rotation of the plunger relative to the columns, the connecting member having means to permit limited axial movement of one plunger relative to the remaining plungers so as to permit tilting of the first wheelarbor.

18. A bandmill as claimed in claim 1 further including:

e. upper and lower stops cooperating with the carrier arm to limit rotation of the carrier arm relative to the yoke assembly.

19. A bandmill as claimed in claim 14 in which one portion of the air spring is moveable transversely relative to the remaining portion to permit adjustment for concentricity between the portions. 

1. A bandmill having spaced coplanar first and second wheels adapted to carry and drive an endless saw, the wheels mounted for rotation about axes coplanar with a central longitudinal plane normal to diametrical planes of the wheels, the bandmill having a column supporting a strain mechanism adapted to move the first wheel within the longitudinal plane relative to the second wheel to strain the saw, the strain mechanism having a first portion including: a. a yoke assembly having a guide slideable within the column, and restraining means cooperating with the yoke assembly to limit movement of the yoke assembly to motion parallel to the longitudinal plane with negligible rotation relative to the column, the yoke assembly having a hinge portion, b. a jack extending between the column and the yoke assembly, the jack adapted to move the yoke assembly relative to the column, c. a carrier arm carrying the arbor of the first wheel, the carrier arm having a hinge portion complementary to the hinge portion of the yoke assembly so as to form a hinge to journal the carrier arm to the yoke assembly to permit rotation of the arm relative to the yoke assembly, d. an air spring extending between the carrier arm and the yoke assembly, the air spring communicating with a pressurized gas supply and serving as an extensible and retractible means to rotate the carrier arm relative to the yoke assembly to strain the saw, and to provide an essentially frictionless mounting to accommodate fluctuations in saw loading.
 2. A bandmill as claimed in claim 1 in which: i. the jack has a central longitudinal axis within the longitudinal plane of the bandmill, so that force from the jack is applied to the first wheel arbor in a plane of the arbor to reduce bending forces applied to the yoke assembly.
 3. A bandmill as claimed in claim 1 in which i. the arbor is carried on the carrier arm at a position between the hinge and the air spring.
 4. A bandmill as claimed in claim 1 in which i. the strain mechanism includes an essentially identical second portion coupled so as to cooperate with the first portion, ii. the arbor has each end carried in a respective carrier arm, the first wheel being disposed between the carrier arms.
 5. A bandmill as claimed in claim 1 in which i. the column has a bore having an axis parallel to the longitudinal axis of the bandmill, ii. the guide is a plunger having an axis parallel to the longitudinal axis of the bandmill, the plunger slding in the bore of the column, iii. the restraining means is a key secured to the plunger, the bore of the column having a complementary keyway, the key engaging the keyway to prevent rotation relative to the column, iv. the air spring has an axis of extension and retraction, whicH axis is essentially parallel to the longitudinal plane of the bandmill, the air spring axis being spaced on a side of the arbor remote from the hinge of the yoke carrier.
 6. A bandmill as claimed in claim 1 in which the yoke assembly has: i. a yoke carrier having the guide, the yoke carrier cooperating with the jack extending between the column and the yoke assembly, ii. a yoke member having the hinge portion, the yoke member being releasably secured to the yoke carrier and having adjustment means to provide limited movement of the yoke member relative to the yoke carrier to permit alignment of the arbor of the first wheel with the arbor of the second wheel.
 7. A bandmill as claimed in claim 1 in which: i. the hinge journalling the yoke assembly to the carrier arm has limited-arc spherical bearing to permit tilting of the arbor.
 8. A bandmill as claimed in claim 1 in which the air spring has: i. a first portion having a piston, ii. a second portion having a cylinder, the portions being moveable relative to each other with negligible frictional losses, iii. a flexible diaphragm having an inner portion secured to the piston, and an outer peripheral portion surrounding the inner portion and secured to the cylinder, the diaphragm and cylinder defining a chamber within the air spring, a fold extending between the piston and the cylinder to permit essentially axial limited movement between the portions with negligible diaphragm strain, iv. an input port to conduct gas to the chamber.
 9. A bandmill as claimed in claim 1 in which i. the strain mechanism includes an essentially identical second portion, the jack of which is releasably coupled to the jack of the first portion, ii. the first wheel arbor has each outer end carried in a respective carrier arm, the first wheel being disposed between the carrier arms, and each carrier arm has: iii. inner and outer ends, the inner end being journalled on a respective yoke assembly by spherical bearings, the outer end cooperating with a respective air spring, a portion of the carrier arm between the ends having a bore to accept an outer end of the arbor of the first wheel, iv. pinch bolt means cooperating with the bore of the carrier arm to clamp the respective outer end of the arbor rigidly in the bore of the carrier arm, v. key means cooperating with the bore of the carrier arm and the arbor to prevent rotation of the arbor relative to the carrier arm, so that the carrier arms and arbor form an essentially rigid assembly capable of withstanding forces from the strain mechanism with negligible relative movement between the arbor and the carrier arms.
 10. A bandmill as claimed in claim 1 in which the yoke assembly has: i. a yoke base having the hinge portion and the guide extending therefrom, the yoke base cooperating with the jack extending between the column and the yoke assembly, ii. the hinge has eccentric adjustment means to provide limited movement of the carrier arm generally normal to the central longitudinal plane of the bandmill to permit alignment of the wheels.
 11. A bandmill as claimed in claim 10 in which i. the hinge has a hinge pin having an eccentric cam spaced between co-axial aligned outer ends, ii. the hinge portion of the yoke base has two spaced hinge members having aligned bores to accept outer ends of the pin and being spaced sufficiently to straddle the eccentric cam, iii. the hinge portion of the carrier arm has a hinge bore generally complementary to the eccentric cam, the hinge bore and cam journalling the arm, so that rotation of the hinge pin rotates the cam to move the hinge axis of the carrier arm relative to the hinge portion of the yoke base.
 12. A bandmill as claimed in claim 11 in which: i. the eccentric cam has a partially spherical bearing surface, ii. the hinge bore of the carrier arm has an internal partially spherical bearing surface complementary to the eccentric cam to provide a limited arc spherical bearing to permit tilting of the arbor.
 13. A bandmill as claimed in claim 1 in which: i. the air spring has a central datum plane when in a mid-stroke position, ii. the hinge has a central axis about which the carrier arm rotates, the central axis of the hinge being generally within the central datum plane of the air spring when the hinge is disposed centrally relative to the yoke assembly, iii. the first wheel arbor has a central axis about which the wheel rotates, the central axis of the wheel arbor being generally within the central datum plane of the air spring when the air spring is at the midstroke position.
 14. A bandmill as claimed in claim 8 in which: i. the piston of the air spring has a piston datum face within a central datum plane of the air spring when at a mid-stroke position, the inner portion of the diaphragm being secured to the piston datum face, ii. the cylinder of the air spring has a cylinder datum face surrounding the piston face and being generally co-planar with the piston datum face when the air spring is at the mid-stroke position, the outer peripheral portion of the diaphragm being secured to the cylinder datum face, iii. the hinge has a central axis about which the carrier arm rotates, and when the hinge is centered so that the hinge is disposed centrally relative to the yoke assembly, the central axis of the hinge is generally within the central datum plane of the air spring, iv. the first wheel arbor has a central axis about which the wheel rotates, the central axis of the arbor being within the central plane of the air spring when the air spring is at the mid-stroke position.
 15. A bandmill as claimed in claim 14 in which: i. the piston has a piston rod having a bore having inner and outer ends, the inner end of the bore serving as the input port and extending inwards from the piston face, and the outer end of the bore communicating with the pressurized gas supply, ii. the diaphragm has a bore adjacent the central portion communicating with the inner end of the bore of the piston rod to admit gas into the chamber defined by the diaphragm and the cylinder.
 16. A bandmill as claimed in claim 9 in which the key means includes: i. a key being a segment of a cylinder having a plane chord face and a convex face, the faces being defined by common spaced parallel side edges and spaced parallel inner and outer end edges, spacing between the side edges and end edges defining chord length and axial length respectively of the faces, the convex face having a radius generally equal to radius of the outer end of the wheel arbor, the key having a bracket adjacent the outer end edge of the face, the bracket cooperating with the carrier arm to prevent rotation of the key, ii. an outer end of the arbor has a flat face having a chord length and an axial length generally equal to the chord length and axial length respectively of the key, the key being complementary to the outer end of the arbor so as to be within a cylindrical envelope of the arbor when the flat face of the arbor contacts the chord face of the cotter piece, the cylindrical envelope enclosing the key and arbor being received in the bore of the carrier arm so that when the key is secured to the carrier arm and the pinch bolt means is secured, the arbor is restricted against rotation relative to the carrier arm to form a rigid assembly.
 17. A bandmill as claimed in claim 1 in which: i. the strain mechanism includes an essentially identical second portion, the jack of which is releasably coupled to the jack of the first portion, ii. the first wheel arbor has each outer end carried in a respective carrier arm, the first wheel being disposed between the carrier arms, iii. each column has a bore having an axis parallel to the longitudinal axis of the bandmill, iv. each guide is a plunger having an axis parallel to the longitudinal axis of the bandmill, the plunger sliding in thE bore of the respective column, v. the restraining means is a connecting member extending transversely between the plungers, the member following concurrent equal movements of the plunger and preventing rotation of the plunger relative to the columns, the connecting member having means to permit limited axial movement of one plunger relative to the remaining plungers so as to permit tilting of the first wheel arbor.
 18. A bandmill as claimed in claim 1 further including: e. upper and lower stops cooperating with the carrier arm to limit rotation of the carrier arm relative to the yoke assembly.
 19. A bandmill as claimed in claim 14 in which one portion of the air spring is moveable transversely relative to the remaining portion to permit adjustment for concentricity between the portions. 